Glasses, such as Pockels glasses, having a zero stress-optic coefficient can be used as components in projection televisions or display apparatuses. A Pockels glass is a glass with a certain lead content which should exhibit, under load, no birefringence in monochromatic light. Most glasses, when exposed to external mechanical, or permanent or temporary thermogradiant forces, become depolarized and scatter light. Pockels glasses which have a relatively high lead content do not become birefringent when exposed to these outside forces, and thus are highly desirable for use in certain optical devices.
However, Pockels glasses such as SF 57 suffer several disadvantages. Their high lead content results in the glass having low chemical durability. As a result, these glasses fail to be stain or climate resistant and must be stored or shipped with a desiccator to prevent degradation. This susceptibility to chemical degradation creates hazards when shipping components from overseas locations. Not only must desiccators be used during shipping and storing, but also when being handled and processed by conventional grinding and polishing techniques. Furthermore, exposure to atmospheric conditions may result in the premature deterioration of the glass components. Another disadvantage of SF 57 is that it has a relatively high market price compared to other conventional glasses, e.g., about 4.5 times more expensive than a standard optical glass, BK 7 (Schott Glass Technologies Inc. of Duryea, Pa.).
Attempts to replace the lead in the SF 57 glass with barium to obtain a glass with a minimal stress-optic effect, improved chemical durability, and lower manufacturing costs have been unsuccessful. Furthermore, attempts to simply remove the lead oxide for improving the chemical durability, and lowering the manufacturing costs of low stress-optic coefficient glass have also been unsuccessful because removal of the lead oxide results in a corresponding increase in the stress-optic effect.
Therefore, it would be desirable to produce a glass with a good chemical durability, and a lower manufacturing cost with a minimal stress-optic effect.